The present invention relates to a transmission apparatus, a reception apparatus, a transmission method and a reception method, which are used for transmission and reception of encrypted data protected against illegal decryption of the encrypted data by a third party capable of inferring an encryption algorithm by decryption of a pattern having a known pre-encryption value, and relates to a recording medium for recording the encrypted data.
In many cases, processing to encrypt data is carried out before transmission of the data whose leakage to outsiders is undesirable. Examples of such data are data requiring protection of a copyright, secret data and private data including privacies.
In a transmission of a program comprising a continuous data stream such music and video data, for example, the data stream is encrypted prior to the transmission due to a need for protection of a copyright.
It should be noted that the term program used in this specification is a data set having the form of a continuous data stream. In general, a wide definition of the program thus includes what is called a track corresponding to data representing a piece of music.
FIG. 1 is a block diagram showing a conventional system in which program data is encrypted in a transmission apparatus 101 and then transmitted from the transmission apparatus to a reception apparatus 102.
First of all, clear-text data DT to be transmitted is input to the transmission apparatus 101 in which the data is subjected to an encryption process in an encrypting unit 111, being converted into encrypted data DTs. The encrypted data DTs is output for transmission by a transmission unit 112.
The encrypted data DTs output by the transmission unit 112 is transmitted to the reception apparatus 102 through a transmission line 103 such as an IEEE (the Institute of Electrical and Electronics Engineers)-1394 bus.
In the reception apparatus 102, a reception unit 121 receives the data transmitted through the transmission line 103 and a decryption unit 122 decrypts the data to reproduce the original clear-text data DT.
Since encrypted data DTs is transmitted through the transmission line 103, the confidentiality of the substance of the data DT is protected against a third party even if the third party fetches the data DTs with an evil intention.
When data such as a piece of music is transmitted in a system like the one shown in FIG. 1 for typically protecting a copyright, however, there is a risk of the data's being decrypted to produce an illegal copy.
Assume for example that the data DT of a program to be transmitted is PCM audio data. In the case of PCM audio data, a soundless portion of a stream of the data is an array of zeros. In the case of 1-bit-digital audio data obtained as a result of ΔΣ modulation, on the other hand, the soundless portion is a fixed pattern such as 96h (=10010110).
In the case of program data representing a music track, the beginning and the end of the program are generally soundless. That is to say, a portion immediately preceding a piece of music and a portion immediately following the piece of music each contain soundless data in many cases except a music medley comprising 2 or more linked pieces of music (programs).
Blocks indicated by dotted lines in FIG. 1 include means for encrypting data DT prior to a transmission through the transmission line 103 and means for fetching encrypted data DTs from the transmission line 103.
Normally, it is difficult to analyze the encrypted data DTs in order to obtain the original data DT itself. That is to say, the encrypted data is difficult to decrypt.
It is feared, however, that knowing that the beginning or the end of program data representing a piece of music is soundless data with a known pre-encryption value, an unauthorized third party may extract the portion and decrypt the portion into fixed data on a trial-and-error basis till the fixed data obtained as a result of decryption matches the pre-encryption value in an attempt to infer the encryption algorithm for encrypting the data DT and, hence, to obtain the original data DT.
In addition, at the present time, a service rendered to users to download a piece of music is being implemented. The service downloads a piece of music on an on-demand basis through a public line such as an ISDN (Integrated Standard Digital Network) or an analog line. In general, the user enters information on a desired piece of music to be downloaded via the keyboard of a personal computer, which then transmits the information to a server rendering the service. The information is typically an ISRC (International Standard Record Code), the name of an artist or the title of a song. The name of an artist can be the name of a singer, the name of a performer, the name of a song composer, the name of a song writer or the name of an arranger. Receiving the information, the server searches a hard disc for a piece of music indicated by the information. The server then encrypts the piece of music prior to a transmission to the personal computer. Receiving the encrypted piece of music, the user transmits a credit number or the like to the server. If the server authenticates the credit number, the server transmits a decryption key to the personal computer.
In such a system, however, there is a risk of an unauthorized user's doing illegal operation on the transmitted piece of music. That is to say, such the unauthorized user may probably steal the encrypted piece of music from the network without obtaining a decryption key in order to avoid charging for the music. Then, the user carries out various kinds of work to decrypt a portion of the information on the piece of music on a trial-and-error basis till the result of decryption matches a known pre-encryption value of the portion in an attempt to infer the encryption algorithm. As described above, the information is typically an ISRC (International Standard Record Code), the title of a song or the name of an artist, which can be the name of a singer, the name of a performer, the name of a song composer, the name of a song writer or the name of an arranger.
Of course, once the encryption algorithm is known, the unauthorized user is capable of illegally obtaining the data DT later with ease. That is to say, the unauthorized user is capable of carrying out an illegal deed of infringing a copyright.
Thus, in a broad range of fields including transmission of data between apparatuses, transmission of data for recording the data into recording media and transmission of music data in a music distribution system using a public line, there is a risk described above so that it is necessary to develop a technology for avoiding illegal decryption of transmitted data requiring protection of a copyright such as music data.